This invention relates to optically readable data storage media and, more particularly, to techniques to render said media unreadable after being read at least once.
It is often desirable when distributing software or other information, such as music and films, that is recorded on a medium to insure that only one party is enabled to access the recorded information. For example, a company that sells computer software will find it advantageous to enable only the purchaser to read the software from a disk and transfer or install the software to computer memory, such as a hard disk, while preventing subsequent access by other parties to the software.
It would also be advantageous when renting content on an optical media, such as a DVD or a CD, to provide only a limited amount of time during which the content can be viewed or otherwise accessed, and to then prohibit further viewing or access (referred to herein as a limited-play mechanism). In this manner the person renting the media need not return the media, as after the limited amount of time expires the media becomes unusable.
Successful readout of an optical disk by all current disk readers heavily relies on a number of parameters that characterize the readout laser beam on its path from the laser to the reflective data layer of the disk and back to the optical pickup system of the reader. The electromagnetic wave structure of the readout beam is described by intensity, phase, polarization, and wave vectors of the wave components that constitute the readout beam. The wave structure of the beam determines geometrical and propagation parameters of the beam, such as beam size, angle of incidence, and angle of convergence.
In addition to reading the data layer of a disk, the reader performs other functions, which are as critical for playability as the data readout itself. These functions include auto-focusing, auto-tracking and error correction. The first two functions allow the reader device to actively control motion of the laser head and spindle motor in order to maintain the required position of the beam relative to the disk. Therefore, maintaining the integrity of the wave structure of the beam throughout its path inside the disk material is a key not only to the successful detection and decoding of the information carried by the beam, but also for the continuity of the readout process.
U.S. Pat. No. 5,815,484 discloses an optical disk having a reflective metallic layer with a plurality of data structures (provided in the form of pits and lands) and a reactive compound superimposed over at least some of the data structures. The reactive compound is a photochromic compound which changes from an optically transparent condition to an optically opaque condition when subjected to readout light and/or atmospheric oxygen. When the compound becomes opaque it prevents readout light from being detected by the readout apparatus, thereby effectively rendering the optical disk unreadable.
A significant perceived disadvantage of this approach is that manufacturing, processing and storage of the disks would require an oxygen-free environment. In addition, the coating materials should be degassed to an oxygen-free state and maintained in this condition.
Another disadvantage is that most of the chemical moieties described have poor light fastness, which would allow a limited play disk to be photobleached and converted to a permanent play disk.
Furthermore, in some cases it may be possible to remove a layer of the photochromic compound, thereby defeating the purpose of providing same on the media.
In general, the unauthorized removal of a layer on the media in an attempt to defeat the limited-play mechanism, and thus extend the useful life of the media, is undesirable.
Reference may also be had to commonly assigned U.S. Pat. No. 6,011,772 for disclosing the use of a barrier layer, the removal of which initiates the action of a reading-inhibit agent to prevent machine reading of information encoding features on an optical disk. The reading-inhibit agent may also be activated by exposure to optical radiation, or by rotation of the disk.
It is a first object and advantage of this invention to provide an improved system and method to render an optically readable media, such as, but not limited to, a laser disk, a compact disk (CD), or a digital video disk (DVD), unreadable.
It is a second object and advantage of this invention to provide an improved system and method to render an optically readable media permanently unreadable, after having been read at least once.
It is a third object and advantage of this invention to provide an improved system and method to prevent or discourage tampering with a layer intended to limit the useful life of the optically readable media.
It is still another object and advantage of this invention to provide a mechanism that relies on evaporation of a substance for modifying the optical properties of an optically readable media so as to render the media optically unreadable.
It is still another object and advantage of this invention to provide a mechanism that relies on sublimation of a substance for modifying the optical properties of an optically readable media so as to render the media optically unreadable.
It is another object and advantage of this invention to provide a technique that increases optical scattering from a surface or layer of an optically readable media so as to render the media optically unreadable, and/or to defeat an attempt to remove a limited-play mechanism of the media.
It is another object and advantage of this invention to provide a mechanism that alters a surface characteristic of an optically readable media so as to detrimentally affect a readout apparatus tracking process during an attempted readout of the media.
It is a further object and advantage of this invention to provide a mechanism that causes surface topography changes to an optically readable media so as to detrimentally affect a readout apparatus feedback and tracking process, thereby adversely affecting the fidelity of the readout.
The foregoing and other problems are overcome and the objects and the advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for making an optically readable media unreadable, as is a media that operates in accordance with the method. The method has steps of (a) providing the media with a readout surface layer that is non-flat (i.e., textured in some way, such as by embossing, scratching, depositing a non-planar layer or regions, such as droplets) and that inhibits optical readout of the media; (b) applying a coating layer to the non-flat surface layer to smooth the non-flat surface and to enable optical readout of the media; and, after an initiation of a readout period, (c) reducing a thickness of the coating layer so as to expose the non-flat readout surface layer, thereby inhibiting optical readout of the media. Preferably, the coating layer is substantially index matched to the readout surface layer. In one embodiment the step of reducing the thickness includes a step of evaporation or sublimation of at least one component material of the coating layer.
Further in accordance with these teachings a method is disclosed for discouraging tampering with a readout-limiting mechanism of an optically readable media, as is a media that is constructed so as to operate in accordance with the method. In this embodiment the method has steps of: (a) providing the media with at least one layer that embodies the readout-limiting mechanism; (b) providing a textured surface that forms part of a layer that underlies the at least one layer, where the textured surface inhibits optical readout of the media; and (c) applying at least one coating layer over the textured surface to smooth the textured surface and to enable optical readout of the media, wherein removal of the coating layer exposes the textured surface thereby inhibiting optical readout of the media. The readout-limiting mechanism can include a layer that changes from an optically transparent state to a state that inhibits successful readout of data structures located on the media or, more generally, a layer that changes from a first state that enables successful readout of data structures located on the media to a second state that inhibits successful readout of data structures located on the media. The readout-limiting mechanism can function as well as one of the coating layers.
The above-mentioned change to the second state from the first state can be induced by an increase in optical absorption and/or by an increase or a change in mechanical stress in a layer or layers, and/or by an increase in optical scattering, and/or by evaporation of a volatile compound, and/or by sublimation of a volatile compound. A volatile compound or material as employed herein is one that is capable of supporting or participating in a material transport driven by a concentration gradient between the material and a surrounding medium, examples of which are evaporation and sublimation. Stated differently, a volatile compound or material is one that is capable of losing mass through a transport mechanism to a surrounding medium.
In a further embodiment of this invention an optically readable media is constructed to include a surface region that encodes information that is optically readable by a reading device and a transparent readout surface layer that contains localized regions each containing a volatile compound having an index of refraction at the readout light wavelength that is closely matched to the material of the surrounding readout surface layer. The localized regions are responsive to an initiation of a readout period for losing at least some of the volatile compound, such as by evaporation or sublimation, for creating voids within the transparent readout surface layer. The presence of the voids increases the optical scattering and thereby inhibits optical readout of the media.
It should be noted that in the embodiments that employ surface roughness or voids, that the optical transmission through the layer can be essentially unchanged.
In various ones of the embodiments of this invention a barrier layer can be provided to protect and to inhibit or regulate the loss of the volatile substance to the surrounding medium, effectively isolating a layer containing the volatile substance from the surrounding medium. The barrier layer could be provided as a peel-off layer for isolating the volatile substance from the surrounding medium at least until it is desired to use the media in the readout device. The media can be packaged in a sealed container, such as a plastic bag, that contains a source of the volatile compound, thereby inhibiting loss of the volatile compound from the media until the media is removed from the container and exposed to the surrounding medium.
The limited play disk is preferably packaged in a sealed container with a source of an agent that inhibits transport of a volatile species from the color forming layer or from a layer that sublimates, wherein the onset of the limited play period coincides with the opening of the container.
It should be understood that the foregoing and other various aspects of the teachings of this invention may be implemented alone or in combination to achieve the purposes set forth herein.